I have to say, as much as I like this exercise, it can easily lead to a shoulder injury if you are pushing those last reps in your last set. Which Is why now I prefer to do Arnold presses because its easier on the joints.

So why would one do this, as opposed to a military press? It's a bit against my weight lifting religion to do sitting what I can do standing, but if there's a good reason to do this version instead (assuming there are no pathologies that would argue one way or the other), I'd love to hear it.

The standing overhead press uses more muscle groups than the seated version. You have to use your abs and legs more to keep your balance, the military version is with your feet together, which makes keeping your balance even harder.

If im not a be begginer what would be the right amount of reps per set. What will be the adquate weight? I've been working out for 4 years now and I really want to get to that next level. Thanks for your help!

Best thing to do is just start light, see what you can do and then adjust your workout from there. Keep a journal of your workouts for a while (if not always) so that you can look back and see what you did last time. Take notes when you're working out and write down if it was too light, just right or too heavy. Having a workout partner to spot you and help you through those last few reps that you just can't quite obtain is a great help too. AND DON'T neglect your legs! :-)